Cancerous microflora
Breast tumor has become a record holder for bacterial diversity
Polina Loseva, N+1
Microbiologists compared a set of intracellular bacteria in a thousand tissue samples belonging to seven different types of tumors. Each of them had its own microbiome, and most often the bacteria were found inside the tumor cells themselves. The composition of bacteria turned out to be different not only depending on the organ in which the tumor grew, but also on the properties of the neoplasm itself – for example, whether it depends on hormones and whether it reacts to immunotherapy. Perhaps in the future, this knowledge can be used to influence the tumor through its bacteria – just as doctors are now trying to treat patients through their intestinal microbes. The work was published in the journal Science (Nejman et al., The human tumor microbiome is composed of tumor type–specific intracellular bacteria).
Contrary to early ideas, there are practically no sterile parts in the human body. Through the skin, blood, intestinal wall and other mucous bacteria get to the most different parts of the body. And they are not always there in place: microorganisms are often attributed a role in the development of tumors. Moreover, they can act both indirectly (for example, when intestinal microbes affect carcinogenesis in other parts of the body) and directly (for example, the microbiome of a breast tumor).
To understand how bacteria are associated with carcinogenesis and how they can help doctors in the fight against tumors, a group of researchers led by Ravid Straussman from the Weizmann Institute in Israel decided to make a collective portrait of intracellular bacteria from different human organs. Scientists collected 1010 tissue samples from tumors of the breast, lung, ovary, bone and pancreas, as well as melanoma and glioblastoma (neoplasm from auxiliary cells of nervous tissue in the brain).
The researchers immediately noticed that there could be quite a few intra–tumor bacteria, so they developed a complex system of controls so that it would not turn out that their measurements were the result of contamination of the sample. In addition to experimental samples, they took 516 samples of healthy tissue, including adjacent to the tumor, 643 negative controls – a sterile solution that went through all laboratory procedures, and 168 more controls from clinics – pieces of paraffin, which were treated in clinics the same way as if it were real tissue. Finally, traces of bacteria in the samples were also determined in several ways at once: the concentration of bacterial DNA, RNA, as well as lipopolysaccharide and lipoteichoic acid, components of the cell wall that do not exist in the body of animals, was calculated.
At the first stage of the work, it turned out that bacteria occur unevenly in tumors – for example, in only 14.3 percent of melanomas, but in more than 60 percent of tumors of the mammary and pancreatic glands and even bone, which does not contact the mucous membranes in any way. Upon closer examination, it turned out that bacteria are most often found inside cells, both cancer and immune. Lipopolysaccharide was more often found in the cytoplasm of tumor cells, and lipoteichoic acid was found in macrophages. In the second fact, however, there is nothing surprising – macrophages work as scavengers in any inflamed tissue, so they absorb bacteria every now and then, and they sometimes turn out to be resistant and remain living inside them.
The authors then sequenced the ribosomal RNA of the bacteria to assess the diversity of species. At the first stage, they found a total of 9190 bacteria in all samples, but after applying filters that eliminated traces of infection in the laboratory and clinic, as well as excluding widespread species, there were only 528 bacterial species that were found only in a certain tumor or tissue adjacent to it. Among them, the breast cancer microbiome turned out to be the most diverse, there were a total of one and a half hundred different genera of bacteria, and in each sample there were an average of 16.4 species. In second place is a lung tumor (if we count by the total number of births) and bones (if measured by the average number of different species in the sample), in last place is melanoma. At the same time, in the case of the record holder – breast tumors – pathological samples turned out to be carriers of a more diverse microbiome than fragments of healthy tissue.
The number of bacterial fragments in 40 nanograms of DNA from the tumor. The numbers at the top indicate the proportion of tumor samples in which this DNA is found. Drawings from the article by Nejman et al.
The presence of bacteria in tumors. From left to right: staining with hematoxylin-eosin (general view of the tumor), staining with lipopolysaccharide and bacterial RNA. From top to bottom: breast tumor, lung tumor, melanoma, glioblastoma.
Then the researchers asked the question: are there really living bacteria in the tumors, and not their remains? To find out, they collected five fresh breast cancer samples and sowed them on different nutrient media. Of these, only in one case only 37 bacterial colonies grew, and in all the others – more than a thousand. It turned out that they belong to 37 different species, 11 of which were already found in the sequencing results, the rest could not be determined, or they did not pass a strict filter system and could be the result of contamination. In general, representatives of three main types of bacteria – Proteobacteria, Firmicutes and Actinobacteria - were found inside the tumors.
Comparing the set of inhabitants of different tumors, the authors of the work noticed that each has its own characteristics: different samples of tumors of the same type were more similar than samples of tumors of different types. For example, in pancreatic cancer, proteobacteria were more common (probably swam there from the duodenum), and actinobacteria were more often found in non-intestinal tumors. And the ratio of representatives of Proteobacteria and Firmicutes turned out to be different depending on the tumor: in glioblastoma, for example, it is much higher than in melanoma. Differences were found even in different types of breast tumors – the set of bacteria turned out to be different depending on whether this tumor was hormone-dependent or hormone-independent. In general, the microbiome of the tumor tissue turned out to be more diverse than the microbiome of adjacent tissues, although in general the set of bacteria in most cases was similar.
Different types of tumors can be distinguished by the ratio of bacteria in their composition.
Finally, the researchers suggested that a set of bacteria can predict the characteristics of both the tumor and its host. So, they found out that the intracellular microbiome differs in smokers with lung cancer and patients who do not smoke: the former have much more bacteria capable of decomposing acrylonitrile, aminobenzoates and other substances from tobacco smoke. And the microbiome of melanoma in patients differed in 46 positions, depending on whether they reacted to immunotherapy or not.
Thus, from studies of the human microbiome as a whole, scientists have moved on to the study of the microbiome of individual tumors. Their work does not allow us to establish a causal relationship: whether the bacteria themselves provoke the development of tumors or come after, when a favorable environment arises in it. It is also unknown in which direction the migration takes place: bacteria from the surrounding tissue colonize the tumor or vice versa. However, it is now obvious that the set of microbes is closely related to the properties of the tumor. And this allowed the authors of the work to assume that someday we will learn how to manipulate the intra-tumor microbiome as well as the intestinal one: to feed "useful" types of bacteria, kill "superfluous" with antibiotics or transplant the entire community in order to make the tumor succumb to certain therapies.
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